name like Katharine nn Kat...

My 2nd daughter is Katharine and we call her by both her full name and Kat. It's really worked well and I'd like to find another girls name with a similar feel. Katharine is of course classic and Kat is a bit fun and unexpected. Also Kat and Katharine interchange really easily and I like that. (Her siblings all have 1-syllable names - in my siggy- so I'd like the next baby's name to match Katharine's name more than the others.)